1. Field of the Invention
This invention pertains to a method and apparatus for thermally developing a photosensitive element, and particularly to a method and apparatus that removes a development medium from the photosensitive element at a location away from where the development medium is contacted to the photosensitive element.
2. Description of Related Art
Flexographic printing plates are well known for use in printing surfaces which range from soft and easy to deform to relatively hard, such as packaging materials, e.g., cardboard, plastic films, aluminum foils, etc. Flexographic printing plates can be prepared from photosensitive elements containing photopolymerizable compositions, such as those described in U.S. Pat. Nos. 4,323,637 and 4,427,759. The photopolymerizable compositions generally comprise an elastomeric binder, at least one monomer and a photoinitiator. Photosensitive elements generally have a photopolymerizable layer interposed between a support and a coversheet or multilayer cover element. Upon imagewise exposure to actinic radiation, photopolymerization of the photo-polymerizable layer occurs in the exposed areas, thereby curing and rendering insoluble the exposed areas of the layer. Conventionally, the element is treated with a suitable solution, e.g., solvent or aqueous-based washout, to remove the unexposed areas of the photopolymerizable layer leaving a printing relief which can be used for flexographic printing. However, developing systems that treat the element with a solution are time consuming since drying for an extended period (0.5 to 24 hours) is necessary to remove absorbed developer solution.
As an alternative to solution development, a “dry” thermal development process may be used which removes the unexposed areas without the subsequent time-consuming drying step. In a thermal development process, the photosensitive layer, which has been imagewise exposed to actinic radiation, is contacted with an absorbent material at a temperature sufficient to cause the composition in the unexposed portions of the photosensitive layer to soften or melt and flow into an absorbent material. See U.S. Pat. No. 3,060,023 (Burg et al.); U.S. Pat. No. 3,264,103 (Cohen et al.); U.S. Pat. No. 5,015,556 (Martens); U.S. Pat. No. 5,175,072 (Martens); U.S. Pat. No. 5,215,859 (Martens); and U.S. Pat. No. 5,279,697 (Peterson et al.). The exposed portions of the photosensitive layer remain hard, that is do not soften or melt, at the softening temperature for the unexposed portions. The absorbent material collects the softened un-irradiated material and then is separated/removed from the photosensitive layer. The cycle of heating and contacting the photosensitive layer may need to be repeated several times in order to sufficiently remove the flowable composition from the unirradiated areas and form a relief structure suitable for printing. After such processing, there remains a raised relief structure of irradiated, hardened composition that represents the irradiated image.
Processors for thermal development of flexographic printing elements are known. U.S. Pat. No. 5,279,697 describes an automated process and apparatus for handling an irradiated printing element and accomplishing repeated heating and pressing to remove the unirradiated composition from the element. WO 2001/18604 also describes a method and apparatus for thermal processing a photosensitive element. In both thermal processing apparatuses the absorbent material is a continuous sheet of a web, typically a non-woven, which is pulled from a supply roll and passed over a hot roll to heat the web. The hot roll is urged toward a drum carrying the photosensitive element, thereby pressing the heated web against an exterior surface of the photosensitive element. Heat is transferred by conduction from the hot roll, through the absorbent web, to the photosensitive element upon contact so the temperature of the composition layer is raised sufficiently to enable the unirradiated portions of the composition layer to liquefy and be absorbed into the absorbent web. As the drum and hot roll rotate in contact together, the web is pressed against the photosensitive element to absorb the liquefied unirradiated composition and is then pulled away from the element, thereby separating the absorbed composition from the photosensitive element. WO 2001/18604 indicates that due to heat transfer characteristics of the photosensitive element it is necessary to separate the web from the element immediately after absorption. This is accomplished by pulling the web away from the element just after the nip in a direction of about 90 degrees from the direction of travel of the element as the web continues its wrap about the hot roll. The web is transported away from the hot roll and rewound on to a take-up roll. Several cycles of passing the photosensitive element past the hot roll are repeated to progressively remove the unirradiated composition from the element.
A problem arises in these thermal development processors in that the removal of the absorbent web from the still warm photosensitive element can induce defects in the resulting relief element. In actual operation of the thermal processors as described above, the web separates from the photosensitive element at any location in a substantially triangular-shaped region after the nip. The substantially triangular-shaped region is located adjacent the hot roll and drum and has a first vertex after the nip, along the surface of the drum to a second vertex located about where the photosensitive element is in its lowest position on the drum, and along a take-up side of the hot roll to a third vertex which extends into a space below and beyond the hot roll. This substantially triangular-shaped region is approximately (10 inch) by (3-6 inch) by (6-10 inch) ((25.4 cm) by (7.6-15.2 cm) by (15.2-25.4 cm)) in cross-section. The location at which the web separates from the element can uncontrollably vary in the described triangular-shaped region, and also along the width of the element, depending upon several factors, such as, the relief image being formed, the thickness of the plate, the thermal development conditions including pressure, temperature, cycle, web speed, nip geometry, and flux of heat, etc. The separating web, which is under velocity control in commercial thermal processors, can cause the element to pass through the nip faster than expected, i.e., slip through the nip. As such, the photosensitive element can lift off the drum or sag and separate from the drum surface after the nip into the described region as the web is being pulled away from the photosensitive element. Although the drum includes a tacky surface to hold the plate to the drum, the degree of tack at times can be insufficient to prevent the element from slipping through the nip. Uncontrolled separation of the web and the lifting or sagging of the photosensitive element while the element is still hot, bends the element and induces strains in the structure of the element which creates a defect, called waves, in the resulting relief element. The non-uniform strains imparted in the element while the support is at a temperature higher than the glass transition temperature result in deformations that remain after the element has cooled or returned to room temperature. The deformations are waves of localized distortions resulting in a non-planar topography of the photosensitive element. Because of the uncontrolled nature of the web separation in thermal development of the prior art, waves of distortions can form in different locations in each element processed.
Relief printing forms having waves result in poor print performance. In multicolor printing, when one or more of the relief printing forms have waves the printed image has poor registration. Even in single color printing, waves in the relief printing form may print an image that is not an accurate reproduction of its original, so called image infidelity, by printing straight lines as curves for example. Further, the relief printing form having waves may incompletely print the image due to intermittent contact of the inked surface of the printing form to the printed substrate.